Re: [PATCH] libacpi: Remove CPU hotplug and GPE handling from PVH DSDTs

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 10.09.25 18:16, Alejandro Vallejo wrote:
> On Wed Sep 10, 2025 at 5:02 PM CEST, Jan Beulich wrote:
> > On 10.09.2025 16:49, Alejandro Vallejo wrote:
> > > CPU hotplug relies on the guest having access to the legacy online CPU
> > > bitmap that QEMU provides at PIO 0xAF00. But PVH guests have no DM, so
> > > this causes the MADT to get corrupted due to spurious modifications of
> > > the "online" flag in MADT entries and the table checksum during the
> > > initial acpica passes.
> > I don't understand this MADT corruption aspect, which - aiui - is why
> > there's a Fixes: tag here. The code change itself looks plausible.
> >
> > Jan
> When there's no DM to provide a real and honest online CPU bitmap on PIO 0xAF00
> then we get all 1s (because there's no IOREQ server). Which confuses the GPE
> handler.
>
> Somehow, the GPE handler is being triggered. Whether this is due to a real SCI
> or just it being spuriously executed as part of the initial acpica pass, I don't
> know.
>
> Both statements combined means the checksum and online flags in the MADT get
> changed after initial parsing making it appear as-if all 128 CPUs were plugged.
>
> This patch makes the checksums be correct after acpica init.
>
> Grygorii noticed the checksum mismatch while validating an ACPI dump on a PVH
> Linux system.
Below is "acpidump -r 0xfc000000" from PVH guest (not dom0) for MADT 
before/after this patch:

Before:

Firmware Warning (ACPI): Incorrect checksum in table [APIC] - 0x59, should be 
0xFFFFFFE3 (20250404/utcksum-208)
APIC @ 0x0000000000000000
    0000: 41 50 49 43 52 00 00 00 02 59 58 65 6E 00 00 00  APICR....YXen...
                                     ^^ incorrect
    0010: 48 56 4D 00 00 00 00 00 00 00 00 00 48 56 4D 4C  HVM.........HVML
    0020: 00 00 00 00 00 00 E0 FE 00 00 00 00 02 0A 00 00  ................
    0030: 02 00 00 00 00 00 01 0C 00 00 00 00 C0 FE 00 00  ................
    0040: 00 00 00 08 00 00 01 00 00 00 00 08 01 02 01 00  ................
    0050: 00 00

After:
APIC @ 0x0000000000000000
    0000: 41 50 49 43 52 00 00 00 02 76 58 65 6E 00 00 00  APICR....vXen...
                                     ^^ correct
    0010: 48 56 4D 00 00 00 00 00 00 00 00 00 48 56 4D 4C  HVM.........HVML
    0020: 00 00 00 00 00 00 E0 FE 00 00 00 00 02 0A 00 00  ................
    0030: 02 00 00 00 00 00 01 0C 00 00 00 00 C0 FE 00 00  ................
    0040: 00 00 00 08 00 00 01 00 00 00 00 08 01 02 01 00  ................
    0050: 00 00

--
Best regards,
-grygorii